Microprocessors, digital signal processors (DSPs) and other synchronous digital logic circuits require a clock signal to maintain synchronization and to control operations. One limitation to the processing power of a processor embodied on an integrated circuit chip is the amount of power the processor can dissipate. Similarly, in portable applications, such as wireless communications, battery capacity can limit the amount of power a chip can consume.
A number of low-power-dissipation oscillator circuits have been proposed or suggested. One popular design is disclosed in U.S. Pat. No. 5,396,195, entitled “Low-Power-Dissipation CMOS Oscillator Circuits,” to Gabara, incorporated by reference herein. The disclosed Gabara Oscillator comprises a pair of cross-connected metal-oxide-semiconductor (MOS) devices and an associated inductor-capacitor (LC) tank circuit. Generally, the drain electrode of the first MOS device is directly connected to the gate electrode of the second MS device and the drain electrode of the second MOS device is directly connected to the gate electrode of the first MOS device.
In addition, a number of techniques have been proposed or suggested for adjusting the frequency tuning range of such low-power-dissipation oscillator circuits. For example, A. Yamagishi et al., Microwave Symposium Digest., 2000 IEEE MTT-S Int'l, Vol. 2, 735-738, (June, 2000) discloses a low-voltage 6-GHz-band CMOS monolithic LC-tank VCO using a tuning-range switching technique. In addition, Liping Zhang and A. A. Sawchuk, Circuits and Systems, 2002. ISCAS 2002. IEEE International Symposium on Circuits and Systems, Vol. 2, II-804-II-806 (May, 2002), discloses a monolithic multi-phase LC-tank VCO.
These representative techniques for adjusting the frequency tuning range of oscillator circuits both employ varactors such that the capacitance is applied directly to the output nodes of the LC tank circuit (which typically contains a certain amount of parasitic capacitance). These parasitic capacitances can be quite large when compared to the capacitance of the varactor. If these parasitic capacitances dominate the capacitance being introduced into the LC tank circuit, then the adjustable frequency range of the varactor control will be reduced. A need therefore exists for methods and apparatus for adjusting the frequency tuning range of oscillator circuits with improved adjustable frequency range.